Protection of application by password

Unless the app itself has a password option built-in to it then you can't - the only restrictions that are available on iOS are those in Settings > General > Restrictions which allows you to hide some of the built-in apps and third-party apps by their age rating, there isn't the ability to password protect individual apps or app folders.

You'll have to buy apps which comes with passcode protection e,g. Quickoffice, GoodReader, Office2 HD, Dropbox